Interesting article in The Daily Telegraph which does in my experience as a family lawyer ring true.
Why are people unfaithful to their wives and husbands? It’s the million dollar question. The obvious answer is sex but according to an article in Psychology Today, sex is rarely the primary motive for infidelity. For both men and women, it's more often about forming emotional connections that are missing from their existing relationships. Occasionally, as is the case with Yuri and Lara in Dr Zhivago, these emotions get out of hand and love comes into the equation. It’s complicated.